802.11 Protocol Architecture
FIGURE 2.24
CF-Ack frame (NO DATA)
CF-Poll frame: The point coordinator sends this frame to a station
granting the station permission to transmit a single frame to any
destination. If the polled station has no frame to send, it must send a
Null Function Data frame. If the sending station does not receive any
frame acknowledgement, it cannot retransmit the frame unless the point
coordinator polls it again. If the receiving station of the contention-free
transmission is not CF Pollable, it acknowledges the reception of the
frame using DCF rules. DCF rules specify that the receiving station
wait one SIFS, and then transmits the ACK frame without regard to its
own NAV.
FIGURE 2.25
CF-Poll frame (NO DATA)
CF-Ack + CF-Poll frame: Used by the point coordinator to
acknowledge a correctly received frame and to solicit a pending frame
from a station.
